A conventional method of sending a plurality of facsimiles to a plurality of facsimile devices is illustrated in FIG. 1. This is a DID system, which is capable of handling incoming telephone calls and facsimiles, but not outgoing calls and facsimiles in the absence of additional lines. It is typically used in an analog (POTS) environment. This technique allows a relatively large number of users 9, 13 to share a relatively small number of telephone lines. Each user (telephones 9 and fax devices 13) is assigned an individual number, for example, NXX-XDDD. Numbers NXX-X000 through NXX-X199 share a bank of, e.g., 40 telephone lines 6; and numbers NXX-X200 through NXX-X220 share a bank of 10 fax lines 10. These lines 6, 10 are unusual in that CO 3 plays the role normally taken by the user 9, 13, that of lifting a receiver and dialing a number; and the user 9, 13 takes the role normally assumed by CO 3, namely, providing a 48 volt battery and a dial tone, and directing calls based upon digits dialed by another.
Calls may be sent to DID recipients 9, 13 from either traditional (POTS) or new technology (ISDN) callers. In both cases, the DID technology is hidden from the caller, since the caller simply dials a NXX-XDDD or NPA-NXX-XDDD number.
Generally, when someone from within PSTN 1 dials a 7-digit DID number, CO 3 looks at the first 4 digits and determines that the particular customer 5 is the desired recipient of the call. CO 3 then arbitrarily selects one of the available lines 6, 10, depending upon whether the dialed number is assigned a voice or facsimile function, respectively. PBX 7 or fax card 11 then gives a dial tone to CO 3; then PBX 7 or fax card 11 connects the caller to the appropriate device 9, 13 based upon the last 3 digits of the telephone/fax number.
In the system illustrated in FIG. 1, 40 telephone lines 6 between the customer premises 5 and CO 3 handle 200 internal telephones 9 connected to customer-installed PBX 7 via 200 telephone lines 8; and 10 fax lines 10 connecting CO 3 and customer-installed fax card 11 handle 20 facsimile devices 13 coupled to fax card 11 via 20 fax lines 12.
The prior art system illustrated in FIG. 1 is limited, particularly for facsimile applications, because:
1. It is usually an analog system rather than a digital one.
2. It requires special customer equipment 7, 11 and non-standard and expensive lines 6, 10 from the telephone company. Telephones 9 and facsimile devices 13 must also be special, because they need to generate 48 volts, plus have the ability to understand DTMF.
3. It does not allow the lines 6, 8, 10, 12 to be used for outgoing calls when they are not being used for incoming calls. Separate lines are necessary.
4. It requires a high degree of maintenance.
In a second method of the prior art, traditional POTS lines which are not DID may be used to route PBX or fax calls if an extension number is appended to a seven or ten-digit telephone/fax number by the caller. In this situation, the caller dials the number, pauses while the PBX or fax card answers, then enters the extension number, generally in response to a dial tone or other prompt such as a recorded voice message. This method is similar to the old-fashioned method of reaching an employee of a company by telling a live or automated PBX attendant the extension desired.
Drawbacks to this method include:
1. Delay.
2. This system may not be able to handle incoming calls from an automated calling device.
3. There are no standard protocols for implementing this system.
In a third system of the prior art, calls between ISDN customers may be routed using the digital sub-addressing feature of ISDN technology to obtain the individual extension of the desired recipient. Digital sub-addressing allows additional digits to be exchanged between customer end points during the "hand-shaking" which occurs between those end points at the beginning of each call. This method, however, has the following limitations:
1. The customers on both ends of the connection must be using ISDN equipment. No provision is made for analog POTS customers calling ISDN recipients.
2. Not all ISDN services as implemented by individual telephone companies allow sub-addressing.